The present invention relates to a spark plug for internal combustion engines.
A long period of usage of a spark plug causes a center electrode and a ground electrode thereof to be damaged by a fire and consumed, thus increasing the spark gap. By this, there may possibly be caused such a case in which a required voltage necessary for producing a spark discharge across the spark gap becomes higher and exceeds a maximum capacity of an ignition system or a spark is not produced across the normal spark gap but across to an end surface of the ground electrode or an end surface of a metallic shell. As a result, the spark plug is disabled to ignite a combustible mixture assuredly, leading to a deterioration of the durability of the spark plug itself.
For this reason, in the prior art spark plug, a center electrode tip made of Pt or a Pt alloy is attached to the axial end of the center electrode to suppress enlargement of the spark gap. In the meantime, the melting point of Pt is about 1800xc2x0 C. After the spark plug having the center electrode tip made of Pt or a Pt alloy is used for a long period, the surface of the center electrode tip is partially melted to cause a granular deposit thereon, and the deposit keeps growing. Such a granular deposit on the surface of the center electrode tip is effective for suppressing enlargement of the spark gap for thereby preventing increase of the discharge voltage and occurrence of a side spark phenomenon (i.e., a phenomenon causing a spark not across a normal spark gap but an abnormal gap). However, since the melting point of the tip made of Pt or a Pt alloy is a little low, the tip encounters a problem that it is damaged by a spark and therefore the electrode consumption suppressing effect is limited to some extent.
Thus, for the purpose of attaining a more efficient electrode consumption suppressing effect than that in the case the tip made of Pt or a Pt alloy is used, it has been proposed a spark plug having a center electrode tip made of Ir or an Ir alloy as disclosed in Japanese Patent Provisional Publication No. 9-219274. Since the melting point of Ir is about 2400xc2x0 C. and therefore higher as compared with that of Pt, the tip made of Ir can attain a better durability and is more efficient for preventing itself from being damaged by a spark as compared with that made of Pt, thus making it possible to elongate the life of the spark plug.
Since the melting point of Ir is higher than that of Pt, the tip made of Ir can effectively be prevented from being damaged by a spark and therefore can attain an improved durability. However, a granular deposit is hardly formed on the surface of the tip. In this connection, in the spark plug having a center electrode tip made of Ir, there is not caused any problem if the center electrode tip is accurately aligned with the ground electrode. However, a certain misalignment may possibly occur in manufacture. If there is such a misalignment, the granular deposit is formed in case of the Pt tip to suppress enlargement of the spark gap and therefore the discharge voltage is stabilized to jump properly across the spark gap to produce a spark. However, in case of the Ir tip, the granular deposit is not formed, thus causing the discharge voltage to tend to rise and making higher the frequency at which a side spark jumping across to the end surface of the metallic shell or the like is caused. As a result, ignition of the combustible mixture is prevented, and the unburned gases are emitted, thus decreasing the efficiency of the engine and badly affecting the environment by the emission of the exhaust gases.
It is accordingly an object of the present invention to provide a spark plug which has a long life and can prevent a side spark phenomenon and a variation of discharge voltage for thereby improving the ignitability and preventing emission of unburned gases.
To accomplish the above object, the present invention provides a spark plug comprising a metallic shell having an externally threaded portion, an insulator disposed within the metallic shell and having an axial through hole, a center electrode disposed within the axial through hole of the insulator, and a ground electrode having a joining end portion joined to the metallic shell and a free end portion having a discharge surface which is opposite to an axial end surface of the center electrode to provide therebetween a spark gap, wherein the center electrode has a center electrode tip forming the axial end surface thereof, the center electrode tip being made of a material containing Ir as a major constituent, and wherein Gxe2x89xa62A+0.5 where G is the spark gap in millimeter(s) and A is the distance in millimeter(s) between a first imaginary line and a second marginal line, the first imaginary line being parallel to an imaginary axis of the spark plug and in a position where it touches a first marginal line for the first time when moved toward the spark gap from a side of the spark gap opposite to the joining end portion along a plane including the imaginary axis and crossing the second marginal line at right angles, the imaginary axis being determined on the basis of an axis of the externally threaded portion, the first marginal line being formed by an axial end surface and a side surface of the center electrode tip, the second marginal line being formed by the discharge surface of the ground electrode and an end surface of the free end portion of the ground electrode.
In case of the spark plug of the type shown in FIG. 3, the above described spark gap represents a minimum distance between the axial end surface of the center electrode tip and the discharge surface of the ground electrode (or the discharge surface of the ground electrode tip in case the ground electrode is provided with the ground electrode tip). However, in case of the spark plug of the type shown in FIG. 6, i.e., of the type wherein the center electrode tip has a tapered portion, the spark gap represents a minimum distance between the tapered portion and the discharge surface of the ground electrode or the ground electrode tip. The spark gap G is usually within the range from 0.7 to 1.5 mm, preferably from 0.7 to 1.3 mm and more preferably from 0.85 to 1.1 mm. It is desirable to set the spark gap G within such a range since by so setting, a bridge across the spark gap due to fuel is hard to be caused, the ignitability is never deteriorated and excessive electrode consumption is never caused.
The above described first marginal line is formed by the axial end surface and the side surface of the center electrode tip. The first marginal line corresponds to an edge of the center electrode tip where there may possibly exist a burr or burrs. In such a case, the burr or burrs are first removed from the edge and then consideration of the first marginal line is made. In the meantime, the side surface of the center electrode tip, which forms the first marginal line, is, for example, cylindrical.
The above described second marginal line is formed by the discharge surface and the end surface of the ground electrode. The ground electrode is formed by cutting a coiled wire of a rectangular cross section, and the end surface of the ground electrode corresponds to a cut surface of the wire. For this reason, there may occur such a case in which the end surface of the ground electrode is not flat but stepped. In such a case, the second marginal line is determined on the basis of an end surface portion of the ground electrode located nearer to the discharge surface. The above describe distance A can be measured by the use of a projector.
In the spark plug of this invention, by determining the spark gap G (mm) and the distance A (mm) so as to satisfy Gxe2x89xa62A+0.5, suppression of electrode consumption due to spark discharge which is an advantage in case the center electrode tip made of Ir or Ir alloy is used at the end portion of the center electrode for forming the spark gap can be attained. As a result, it becomes possible to elongate the life of the spark plug and at the same time it becomes possible to prevent or suppress occurrence of a side spark phenomenon and a variation of discharge voltage which are apprehensions caused when the center electrode tip made of a material containing Ir as a major constituent is used. In the meantime, xe2x80x9ca material containing Ir as a major constituentxe2x80x9d is intended to indicate that, of the constituents of the material, the weight percentage content of Ir is largest but not intended to indicate that the Ir content is equal to or larger than 50 wt %.
The discharge surface of the ground electrode may be provided with a ground electrode tip to form the spark gap for the purpose of suppressing consumption of the ground electrode. In case the ground electrode tip is provided, the distance B between the second marginal line and a second imaginary line is determined so as to be equal to or larger than 0.22 mm, preferably 0.3 mm or larger and more preferably 0.35 to 0.8 mm (refer to FIG. 3), where the second imaginary line is parallel to the second marginal line and in a position where it touches the ground electrode tip for the first time when moved along the discharge surface toward the ground electrode tip from a side of the ground electrode tip opposite to the joining end portion of the ground electrode, namely, the second imaginary line is parallel to the second marginal line, located on the discharge surface of the ground electrode and on a side of the ground electrode tip nearer to the end surface of the ground electrode and touching the ground electrode tip. The distance B smaller than 0.2 mm is not desirable since the ground electrode tip has a possibility of being separated from the ground electrode due to heat to which it is subjected. In the meantime, in case the ground electrode tip is attached to the discharge surface by welding, there may occur such a case in which a fused alloy portion consisting of the constituents of the ground electrode tip and the constituents of the ground electrode is formed around the joint between the ground electrode tip and the ground electrode. In such a case, the first marginal line is determined without consideration of such a fused alloy portion but on the basis of the ground electrode tip itself.
When the ground electrode tip is disposed so to be at least partially overlain by the center electrode tip, it becomes possible to prevent or suppress abnormal consumption of the discharge surface of the ground electrode due to spark discharge for thereby elongating the life of the spark plug. In the meantime, the ground electrode tip can be made of a material containing Ir as a major constituent similarly to the center electrode tip or a Pt alloy such as Ptxe2x80x94Ni and Ptxe2x80x94Ir.
The center electrode tip can be made of Ir or Ir alloy. There is no limitation on the Ir alloy so long as the weight percentage content of Ir is highest. However, it is preferable to make the center electrode tip of Irxe2x80x94Pt, Irxe2x80x94Rh or Irxe2x80x94Y2O3 since such a tip makes it possible to prevent or suppress oxidation-volatilization of the center electrode tip at the more severe temperature for thereby suppressing electrode consumption. Further, in case the center electrode tip is made of an Ir alloy, it is preferable from the electrode consumption preventing point of view that the solidus line in the equilibrium state diagram of the Ir alloy is equal to or higher than 1900xc2x0 C.
The diameter of the center electrode tip is set so as to range from 0.3 to 1.0 mm and more preferably 0.4 to 0.8 mm. In case the diameter of the center electrode tip is smaller than 0.3 mm, the volume of the tip and the surface for forming the spark gap are too small even if the tip is made of Ir or Ir alloy which is a high melting point material, thus causing the center electrode tip to become not so effective of suppressing electrode consumption due to spark discharge. Accordingly, the diameter of the center electrode tip of smaller than 0.3 mm is not desirable. On the other hand, in case the diameter of the center electrode tip exceeds 1.0 mm, the surface of the tip for forming the spark gap is so large unless the spark plug with such a center electrode tip is used for particular purpose, thus causing the heat of a spark produced at the spark gap to be absorbed by the surface of the tip (i.e., a fire extinguishing phenomenon) and causing a possibility of a misfire. Thus, the diameter of the tip larger than 1.0 mm is not desirable.